Several common rechargeable aqueous electrolyte battery technologies generate hydrogen gas at the anode and oxygen gas at the cathode through electrolysis of water in the electrolyte. Such battery technologies include, but are not limited to, lead-acid, nickel metal-hydride, silver oxide-zinc, nickel-cadmium, bromine-zinc, manganese-zinc, and nickel-zinc systems. Unfortunately, the electrochemical reactions that generate the gases compete with electrochemical reactions for storing energy. Thus gas generation reduces the efficiency of the battery for storing energy. In addition, if more Coulombs go toward oxygen (hydrogen) gas generation than toward hydrogen (oxygen) gas generation the cathode (anode) has a lower state of charge (SOC) than the anode (cathode), which can lead to overall poor performance and to battery short-circuiting.
A small portion of the generated gas stays on the electrodes as attached bubbles, but the majority (greater than 95%) of the gas mixes in the common headspace of the battery. A process called recombination can convert the hydrogen and oxygen gas to liquid water which can go back into the electrolyte. Recombiners are commonly placed in the headspace in order to do the gas conversion. They are commonly made of high surface area catalytic materials such as platinum or palladium powder. In sealed, valve-regulated lead acid batteries where absorbed glass cloth holds the electrolyte, recombination can also occur when oxygen gas contacts the anode and when hydrogen gas contacts the cathode. The end result is the same: conversion of hydrogen and oxygen gas to water.
The chemical reaction for a recombiner to convert hydrogen and oxygen gas to water is:2H2(g)+O2(g)←→2H2O(liq)Chemical reaction rates increase monotonically as the concentration of reactants increases and as temperature increases. If either hydrogen or oxygen partial pressure becomes low, below about 3.5 kPa (0.5 psi) for example, the rate of recombination will become very slow.
Current technologies for sealing batteries include relief valves that can vent gases to the environment. When the rate of gas generation is different from that used for stoichiometric recombination of hydrogen and oxygen gas to water, gas pressure in the battery increases. If the gas pressure in the battery becomes too high, the relief valves release gas to the environment, restoring safe pressure levels in the battery. Both increased pressure and release of gas are highly undesirable because increased pressure creates the risk of battery container rupture especially if the relief valve were to fail. Hydrogen and oxygen mixtures are flammable and can be explosive when released to the environment. Other health hazards arise if the hydrogen gas is concentrated enough to act as an asphyxiant or if minor gas components or particulates are released, such as hydrogen sulfide in the case of lead-acid batteries.
Significant effort has been made to maximize the rate of hydrogen and oxygen recombination. If a recombiner can keep gas pressure at safe levels (e.g., between about 7 and 70 kPa (about 1 and 10 psi)), the relief valve is not employed and safety concerns are decreased.
It would be extremely useful if new methods could be found for controlling gas pressures in sealed electrochemical cells so that the pressures remain within safe limits, a balanced state of charge between positive electrodes and negative electrodes is maintained, and battery efficiency is maximized.